Optical devices, for example, OLEDs, may be categorized into a bottom emission structure in which light is emitted toward a glass substrate, and a top emission structure in which light is emitted in a direction opposite to a glass substrate, depending on the emission structure. In the bottom emission structure, a cathode acts as a reflector by using a metal thin film of aluminum or the like, and an anode acts as a path, through which light is emitted, by using a transparent oxide conductive film of indium tin oxide (ITO) or the like. In the top emission structure, a cathode is formed as a multi-layer thin film including a very thin silver thin film, and light is emitted through the cathode. In the field of lighting panels, with the exception of transparent panels in which light is emitted through both surfaces, the bottom emission structure is generally used and the top emission structure is rarely used.
In a laminate used for optical devices such as an OLED, only about 20% of emitted light is emitted externally, and about 80% of the emitted light is lost. This is caused by one of two reasons: a wave-guiding effect due to a difference in refractive indexes among a glass substrate, a transparent electrode and an organic layer; and a total reflection effect due to a difference in refractive indexes between the glass substrate and air.
This is because a planar waveguide is naturally formed in the OLED due to conditions in which a refractive index of an internal organic layer is about 1.7 to 1.8, a refractive index of ITO generally used as a transparent electrode is about 1.9, a thickness of the two layers is about 200 nm to 400 nm (being very thin), and a refractive index of the glass used as a substrate is about 1.5. The amount of light lost by the wave-guiding effect is calculated to be about 45% of the emitted light.
Moreover, since a refractive index of the glass substrate is about 1.5 and a refractive index of external air is 1.0, light which is incident at a critical angle or more when light is emitted externally from the glass substrate causes total reflection and is isolated inside the glass substrate. The amount of the isolated light is about 35% of the emitted light.
As a result, in the laminate used for an optical device, only about 20% of the emitted light is emitted externally due to the wave-guiding effect among the glass substrate, the transparent electrode and organic layer, and the total reflection effect between the glass substrate and air. Since the light emission efficiency of optical devices, such as OLEDs, is as low as the level described above, the external light efficiency of the optical device also remains at a low level.
Accordingly, to enhance the external light efficiency of optical devices, technology for extracting light insolated inside the optical device is needed. Technologies relating to light extraction is progressively drawing much attention as being the core technology that increases efficiency, luminance, and service life of optical devices. External light extraction technology and internal light extraction technology are two types of light extraction technologies. External light extraction technology involves extracting light isolated inside a glass substrate, while internal light extraction technology involves extracting light isolated between an organic layer and ITO.
In the external light extraction technology, technology that adheres a micro-lens array (MLA) film, a light scattering film or the like to the outside of an optical device to enhance light efficiency has been established to a certain degree. However, the internal light extraction technology has not yet reached the level of practical application. The internal light extraction technology is theoretically deemed to be effective for enhancing external light efficiency of optical devices by three times or more. However, since the interface characteristics between an internal light extraction layer and a transparent electrode layer significantly influences the service life of optical devices and the physical characteristics of the material used as the internal light extraction layer significantly influences certain characteristics, such as thermal stability of optical devices, the technology must satisfy electrical, mechanical, and chemical characteristics at all levels, in addition to providing sufficient optical effect.
According to previous studies, an internal light scattering layer, deformation of a substrate surface, a refractive index adjustment layer, photonic crystals, a nanostructure forming method, etc., are known to be effective for extracting internal light. The main objective of the internal light extraction technology is to scatter, diffract, or refract light isolated due to the wave-guiding effect in order to form an incident angle less than or equal to the critical angle, thereby extracting light past an optical waveguide.
Patent Documents 1 to 3 disclose a method that extracts light from the inside by using two materials having different refractive indexes. The patent documents use a base material having a high refractive index (a first refractive index) and a plurality of scattering materials (e.g., air bubbles or precipitation crystals) having a second refractive index different from the first refractive index of the base material included in the base material, as an internal light extraction layer. In the case where a glass frit is used as the base material, and air bubbles are used as the scattering material, a spherical air bubble is formed by way of an internal gap of glass when sintering the glass fit, or by generating a gas such as carbon dioxide (CO2) formed from decomposing a material such as an organic material adhered to a surface of a glass layer when glass is softened. Also, the patent documents use the high priced PD200 substrate, manufactured by Asahi Glass Co., Ltd., as a glass substrate.
Patent Document 4 discloses an internal light extraction layer comprising inorganic phosphor powders as a plurality of scattering materials. When a glass substrate containing alkali metal is used as a glass substrate, alkali metal components are diffused, which affect the characteristics of scattering materials inside a scattering layer. In particular, when the scattering materials are phosphors, the phosphors cannot exhibit its characteristics because the phosphors are weakened by the alkali component. For this reason, Patent Document 4 also uses the high priced PD200 substrate, manufactured by Asahi Glass Co., Ltd., as a glass substrate for preventing alkali metal from being diffused.
Patent Documents 5 and 6 disclose an air bubble, precipitation crystal, particles different from a base material and powdered glass as examples of a plurality of scattering materials, and use a sodalime substrate as a glass substrate.
The methods described in the above patent documents disclose a common feature since an air bubble, which is generated by a gap in sintering a glass fit or generated by oxidizing a material adhered to a surface of a glass substrate, or an additional different material (crystal, phosphor or the like) is used as a scattering material. When the air bubble is used as the scattering material, a strong scattering effect can be obtained. However, since the air bubbles naturally gather at an upper portion of an internal light extraction layer due to their property of rising upwards when they are generated by an internal gap of glass, it is difficult to adjust the density or distribution of the air bubbles to a desired level. Also, when the air bubble is generated by the gap of particles of the glass fit, flexures are formed at a surface of the internal light extraction layer, causing a short circuit in the electrodes. When the air bubble is generated by oxidizing a material adhered to a surface of a glass layer, an additional process is needed to treat the surface of the glass substrate. Furthermore, as in the patent documents, when scattering particles are randomly distributed in the internal light extraction layer, the multi scattering of incident light results in an increase in the loss of light, causing a reduction in the transmittance of visible light.
For this reason, when a gas, especially air, is used as a scattering material, adding an additional scattering material is unnecessary, thereby providing a simple preparing process by which a strong scattering effect can be obtained. However, there still remains a need for a method that concentrates the gases at an interface between the internal light extraction layer and the glass substrate (and not at the upper portion of the internal light extraction layer) when gases are used as a scattering material.